A pyridinium modified β-cyclodextrin: An ionic supramolecular ligand for palladium acetate in C-C coupling reactions in water

Article abstract of DOI:10.1039/c6gc01326k

An ionic Pd(ii) complex stabilized by a water soluble pyridinium modified β-cyclodextrin was prepared and characterized by NMR, mass spectrometry, FT-IR spectroscopy, UV-visible spectroscopy and DLS (dynamic light scattering). The resulting Pd(ii)@Pyr:β-CD complex showed very good catalytic activity in Suzuki-Miyaura and Heck C-C coupling reactions in an environmentally benign water medium. Good to excellent yields were obtained for the coupling of various aryl halides including chlorides with phenylboronic acid/styrene using a catalytic amount of Pd(ii)@Pyr:β-CD. This homogeneous catalyst can be reused and recycled more than six times with only marginal loss of its catalytic activity.

Full text of DOI:10.1039/c6gc01326k

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Pyridinium Modified βꢀCyclodextrin: An Ionic Supramolecular Ligand for
Palladium acetate in CꢀC Coupling Reactions in Water
a
a
,b
Raihana Imran Khan and Kasi Pitchumani*
a
School of Chemistry, Madurai Kamaraj University, Madurai 625 021, India
b
Center for Green Chemistry Processes, School of Chemistry, Madurai Kamaraj University.
Abstract: An ionic Pd(II) complex stabilized by water soluble pyridinium modified βꢀ
cyclodextrin was prepared and characterized by NMR, Mass spectrometry, FTꢀIR spectroscopy,
UVꢀVisible spectroscopy and DLS (dynamic light scattering). The resulting Pd(II)@Pyr:βꢀCD
complex showed very good catalytic activity in Suzukiꢀ Miyaura and Heck CꢀC coupling
reactions in an environmentally benign water medium. Good to excellent yields were obtained
for the coupling of various aryl halides including chlorides with phenylboronic acid/styrene
using a catalytic amount of Pd(II)@Pyr:βꢀCD. This homogeneous catalyst can be reused and
recycled more than six times with only marginal loss in its catalytic activity.
Introduction:
Designing green protocols in chemistry offers lots of scope and challenges for chemists
1
2
in industry and research to develop new products and processes towards societal, environmental
3
and economic benefits. This requires a paradigm shift in approach to reduce the materials and
energy requirements of chemical processes and products, diminish or eliminate the dispersion of
4
ꢀ5
harmful chemicals in the environment, exploit the use of renewable wealth and extend the
1

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robustness and recyclability of products in a way which increases the industrial
6
ꢀ7
competitiveness.
SuzukiꢀMiyaura crossꢀcoupling and Heck reactions are the most widely used protocols
8
for the formation of carbon–carbon bonds and have become significant methods in the synthesis
9
of biaryl and arylolefinic compounds, resulting in a wide range of pharmaceuticals and natural
products. The reagents phenylꢀboronic acids and styrene are readily available, fairly stable and
1
0ꢀ11
form CꢀC bonds
readily and this is one of the key reasons for the keen interest, wide scope
and impressive development of these reactions. Among the various sustainable media, water is
an attractive solvent for chemical reactions due to its low cost, nonꢀflammability, nonꢀtoxicity,
12ꢀ16
and environmental friendliness.
Palladium catalyzed crossꢀcoupling reactions for the formation of carbonꢀcarbon bonds
1
8
are widely used in synthetic and medicinal chemistry for the preparation of new derivatives
1
9
with wide ranging therapeutic potentials .Generally, phosphine ligands are used to complex and
2
0
activate the palladium species. However, they are sensitive to air and moisture, restricting the
reuse of the catalyst and also lead to undesirable residues under aqueous reaction conditions,
2
1ꢀ25
which place significant limitation on their synthetic applications.
of phosphineꢀfree catalytic systems to overcome the above difficulties is considered as one of the
Therefore, the development
2
6ꢀ28
most challenging and demanding areas in organic synthesis.
In this direction, many
2
9ꢀ30
homogeneous catalytic systems have been developed.
Supramolecular chemistry, an emerging area of chemistry, contributes to the
development of numerous innovative concepts. Among the many host systems developed,
cyclodextrins (CDs) as simple naturally occurring macrocyclic oligosaccharides, possess a
hydrophobic cavity that selectively binds substrates and catalyze chemical reactions through
3
1,32
formation of host–guest complexes via noncovalent interactions.
cyclodextrins offers enormous opportunities and challenges for chemists,
Modification of
33ꢀ35
ranging from
achieving solubility in a desired solvent to selective and specific catalysis. Highly water soluble
cyclodextrins are desirable in applications such as drug formulation, controlled release studies
3
6ꢀ38
etc.,
For example, a random conversion of hydroxyl groups to sulfate groups can be easily
3
9ꢀ41
achieved and the product has the desired solubility in water.
Recently perꢀ6ꢀaminoꢀβꢀ
2

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cyclodextrin and imidazolium βꢀcyclodextrin are used in Pdꢀcatalyzed CꢀC coupling
4
2,43
reactions
.
These unique features of cyclodextrins prompted us to report the synthesis,
characterization and catalytic applications of a novel, ionic, pyridinium modified βꢀcyclodextrin
(Pyr:βꢀCD), wherein an octyl chain is linked to the pyridinium moiety attached to the primary
face of a βꢀCD. The synthesized Pyr:βꢀCD (3) is soluble in water and exhibits excellent catalytic
activity in Suzuki–Miyaura and Heck reactions in the presence of palladium acetate. The
observed results are discussed below.
Results and Discussion:
The synthetic pathway to obtain Pyr:βꢀCD (3) is shown in Scheme 1. Monoꢀ6ꢀtosylꢀβꢀ
cyclodextrin (1) was prepared via tosylation from commercially available βꢀCD. Subsequent
−
reaction with Nꢀoctylꢀpyridineꢀ2ꢀamine (2) followed by anionꢀexchange on amberlite Cl resin
afforded the desired amphiphilic pyr:βꢀCD (3) as a pale yellow solid, which was stable towards
oxygen and moisture. The synthesis of (3) was achieved in a single step straightꢀforward
synthesis with 12% yield. The structures of Nꢀoctylꢀpyridineꢀ2ꢀamine (2) and Pyr:βꢀCD (3) were
1
13
confirmed from FTꢀIR, HꢀNMR, CꢀNMR, Mass spectra and Elemental analyses (Figures S1ꢀ
S7, See supporting information).
Scheme 1: Synthesis of pyridinium modified βꢀcyclodextrin (Pyr:βꢀCD).
ꢀ1
In FTꢀIR spectrum (Figure S9), the presence of a band at 3346 cm is due to the hydroxyl
−1
−1
groups of βꢀcyclodextrin and Pyr:βꢀCD (3) shows two bands at 3346 cm and 3537 cm with the
ꢀ1
latter corresponding to secondary NꢀH stretching frequency. A band at 1142 cm corresponds to
3

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the CꢀN stretching frequency of Pyr:βꢀCD (3). The synthesis of Pyr:βꢀCD (3) was also confirmed
from ESIꢀMS spectra (Figure 1a) which shows an m/z peak at 1323.60 corresponding to [M+1
adduct]. In the complexation of Pyr:βꢀCD (3) with Pd(OAc) the peak at 1323.60 disappeared
2
,
and a new peak appeared(Figure. 1b) at 1579.62 (m/z), which corresponds to the adduct of pyr:βꢀ
ꢀ
ꢀ
CD(3)with Pd (cal: m/z = 1580.43 and obs: m/z = 1579.62 for [M+Pd(OAc) +Cl ] ). Formation
2
of Pd(II)@Pyr:βꢀCD is also supported by XRF analysis (Figure S10) and downfield shift of its
1
pyridyl protons in HꢀNMR (spectrum Figure S11).
Figure 1 ESIꢀMS of a) Pyr:βꢀCD (3) and b) Pd(II)@Pyr:βꢀCD complex.
1
To gain further evidence for the formation Pyr:βꢀCD (3), HꢀNMR spectra of Pyr:βꢀCD
(
3), was recorded in DMSOꢀd and of Nꢀoctylꢀpyridineꢀ2ꢀamine (2) in CDCl . (Figures S1 and
6
3
S4). The chemical shifts were assigned by analogy with the chemical shifts of cyclodextrin
4
7
protons reported previously in DMSOꢀd Details of the chemical shifts values are given in
6
.
Table 1. From Figure S4, and Table 1, it is clear that there is considerable downfield shifts for
the alkyl protons (C1 to C7) of Nꢀoctylꢀpyridineꢀ2ꢀamine (2) when it is modified with
cyclodextrin. On the other hand, the chemical shift values of C8 protons undergo an upfield shift.
1
3
Similarly the chemical shift of pyridyl protons also undergoes considerable variations. The Cꢀ
NMR chemical shifts of 3 also undergo significant shifts (Table 1) lending support for binding of
βꢀCD to (2) to form (3). A twoꢀdimensional NOESYꢀNMR experiment was also carried out, with
the aim of examining the potential spatial proximities among the different protons of Pyr:βꢀCD
(3) (Figure S7). Absence of crossꢀcontours, in 2DꢀNOESY spectra, correlating the aromatic
protons of pyridine, aliphatic protons of octyl group and the cyclodextrin protons, indicate that
4

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the pyridine moiety and the octyl side chain were not included into the hydrophobic cavity of βꢀ
CD.
Table 1. Chemical shifts of Nꢀoctylꢀpyridineꢀ2ꢀamine (2) and Pyr:βꢀCD (3).
1
13
Position
HꢀNMR
CꢀNMR
in (2)
in (3)
ꢁδ (3ꢀ2)
in (2)
in (3)
ꢁδ (3ꢀ2)
1
2
3
4
5
6
7
8
9
0.9
2.07
2.50
2.50
2.50
2.50
2.50
2.71
2.87
6.55
6.69
7.50
8.07
+1.17
+1.25
+1.25
+1.25
+1.25
+1.25
+0.89
ꢀ0.69
13.80
22.44
26.72
28.69
28.97
31.59
32.50
44.71
108.26
137.38
141.02
147.76
13.14
18.32
21.69
25.46
32.21
32.21
34.51
35.10
125.71
140.52
145.57
160.71
ꢀ0.66
1.25
1.25
1.25
1.25
1.25
1.82
3.56
6.46
6.59
7.38
7.98
ꢀ4.12
ꢀ5.03
ꢀ3.23
+3.24
+0.62
+2.01
ꢀ9.61
+0.09
+0.10
+0.12
+0.09
+17.47
+3.14
+4.54
+12.95
10
11
12
Compared to native βꢀcyclodextrin, water solubility of pyr:βꢀCD (3) is very high. Figure
S8 shows the solubility of pyr:βꢀCD (3) and a 1:1 mixture of βꢀCD and (2) and the results clearly
indicate the vastly increased solubility of pyr:βꢀCD (3)in water.
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Energy minimization studies (Figs. S13 to S18, SI) were carried out using Insight II
molecular modeling studies to gain an idea about the possible inclusion of octyl side chain of
pyr:βꢀCD (3) in the βꢀCD cavity. From the results shown in Table 2, it is clear that mode A in
which octyl group is present outside the Pyr:βꢀCD (3) cavity is more favored than that of
inclusion of octyl group inside the Pyr:βꢀCD (3) cavity (Mode B).
Table 2: Molecular modeling studies of pyridinium modified βꢀcyclodextrin
a
ꢀ1
Mode of Inclusion
ꢁE (Kcal.M )
Octyl group outside the pyr:βꢀCD cavity
(Mode A)
ꢀ64.8600
Octyl group inside the pyr:βꢀCD cavity.
(Mode B)
ꢀ50.8292
a
Binding energy calculation of modified βꢀcyclodextrin .
The synthesized Pd(II)@Pyr:βꢀCD acts as an efficient catalyst in CꢀC coupling reaction of
different aryl halides/alkenes in Suzuki Miyaura and Heck coupling reactions and the observed
results are discussed below.
i)
Pd(II)@Pyr:βꢀCD catalyzed Suzuki Miyaura coupling reaction in water
The catalytic activity of Pd@Pyr:βꢀCD was evaluated in Suzuki Miyaura coupling
24
reaction in water. As boronic acids are water soluble,
the efficiency of Suzuki Miyaura
reaction in water was hampered by a side reaction of phenylboronic acids, namely
homocoupling. However, the Pyr:βꢀCD (3) supported palladium catalyst with its ionic character,
provides good interaction between water soluble phenylboronic acids and hydrophobic aryl
halides, thus improving the efficiency of Suzuki Miyuara crossꢀcoupling reaction in water.
The coupling between 4ꢀbromobenzaldehyde and phenylboronic acid was chosen as a
model reaction and the results were summarized in Table 3. Without Pd and βꢀCD there is no
reaction (Table 3, entry 1). However, with only Pd or βꢀCD, the yield was very low (entries 2
°
and 3). Among the different solvents examined, water acted as a good solvent at 80 C to yield
the desired products in good yield. Use of various bases were also studied and except K PO ,
3
4
Cs CO and K CO all other bases gave desired coupling products in only moderate yield (entries
2
3
2
3
9
ꢀ16). K PO gave excellent yield compared to Cs CO and K CO . Catalyst loading was also
3 4 2 3 2 3
6

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optimized and in the absence of Pyr:βꢀCD (3), there was no reaction and product formation was
effective at low loading level of Pd(OAc) (0.2 mol%) and Pyr:βꢀCD (3) (0.2 mol%).
2
a,b
Table 3. Optimization of reaction conditions.
°
b
Entry Ligand
Catalyst
Base
Solvent Time (h)
Temp( C) Yield (%)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
ꢀ
ꢀ
K CO
DMF
14h
14h
110
110
110
110
110
110
80
NR
11
19
26
49
46
33
86
46
50
56
56
95
94
2
3
3
3
3
3
3
3
3
ꢀ
Pd(OAc)2 K CO
DMF
2
βꢀCD
Pyr
Pd(OAc)2 K CO
DMF
14h
2
Pd(OAc)2 K CO
DMF
14h
2
Pyrꢀ β CD Pd(OAc)2 K CO
DMF
14h
2
Pyrꢀ β CD Pd(OAc)2 K CO
DMSO
Toluene
Water
Water
14h
2
Pyrꢀ β CD Pd(OAc)2 K CO
14h
2
Pyrꢀ β CD Pd(OAc)2 K CO
14h
80
2
Pyrꢀ β CD Pd(OAc)2 NaOH
14h
80
0
1
2
3
4
5
6
7
8
Pyrꢀ β CD Pd(OAc)2 Na CO Water
14h
80
2
3
Pyrꢀ β CD Pd(OAc)2 Na SO Water
14h
80
2
4
Pyrꢀ β CD Pd(OAc)2 Et N
Water
Water
14h
80
3
Pyrꢀ β CD Pd(OAc)2 K PO
14h
80
3
4
Pyrꢀ β CD Pd(OAc)2 Cs CO Water
14h
80
2
3
Pyrꢀ β CD Pd(OAc)2 K PO
Water
Water
Water
Water
14,6,1, 0.5 h
0.5 h
0.5 h
0.5 h
80
95, 95, 91, 90
95, 95, 92, 88
95, 68, NR
3
4
c
d
e
Pyrꢀβ CD Pd(OAc)2 K PO
80
3
4
Pyrꢀ β CD Pd(OAc)2 K PO
80
3
4
Pyrꢀβ CD Pd(OAc)2 K PO
80
10, 95, 95,
3
4
95, 95, 95
Reaction conditions: Aryl halides (1 mmol, 1 equiv.), phenylboronic acid (1.2 mmol, 1.2 equiv.), base (2 mmol, 2
°
b
c
equiv.), solvent (2 mL), catalyst (0.2 mol %), Ligand (3) (0.2 mol %), 80 C, 0.5 h. GC yield. Yield of the reactions
d
carried out with 50, 40, 30 and 20 mgs of Pd/pyrꢀCD respectively. Yield of the reactions carried out with 100, 50 and
e
0
mol% of base respectively. Yield of the reactions carried out with 0 (0.5), 1 (0.5), 1 (0.3), 1 (0.2), 0.5 (0.2) and 0.2
(
0.2) mol % of ligand (catalyst) respectively.
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The substrate scope of the catalytic system was explored further with a series of aryl
halides and phenylboronic acid under the optimized conditions (0.5 mol % Pd(OAc) at 80 °C in
2
2
mL H O respectively). Different 4ꢀsubstituted aryl bromides containing both electronꢀdonating
2
or electronꢀwithdrawing groups such as methyl, methoxy, amine, cyano, acetyl and fluoride
groups (Table 4, entries 1ꢀ10) yielded the analogous products in very good yields in short
reaction time except nitroꢀgroup substitution (Table 4, entry 9). Heterocyclic bromides like 2ꢀ
and 3ꢀpyridyl bromide and 3ꢀbromothiophene gave moderate yields (Table 4, entries 11ꢀ13).
Nonꢀaromatic as well as aryl bromides containing aliphatic chains also gave good yields (entries
14 and 15). Encouraged by these results, we next turned our attention to aryl chlorides such as
chlorobenzene, 4ꢀchloronitrobenzene and 4ꢀchlorobenzaldehyde. The reactions were facile and
very good yields were observed in 2h (Table 4, entries 16ꢀ18).
Table 4.Suzuki Miyuara reaction of different aryl halides with phenylboronic acid catalyzed by
the pyidinium modified cyclodextrin/Pd(OAc) complex (3).
2
b
Entry
Aryl halide
Product
(Code)
Yield(%)
1
95
Br
2
3
80
72
4
90
5
6
90
91
8

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7
93
8
9
90
74
92
10
1
1
2
90
84
1
1
3
4
89
93
1
15
84
c
1
6
7
88
c
1
90
c
18
87
Aryl/alkyl halide (1 mmol), phenyl boronic acid (1.2 mmol, 1.2 equiv.), K PO (2 mmol), water (2 mL), Pd(OAc)
3
4
[c]
2
°
[b]
(
0.2 mol%), ligand (3) (0.2 mol%), 80 C, unless otherwise noted. GC yield; Reaction time 2h.
9

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Pd(II)@Pyr:βꢀCD catalyzed Heck coupling reaction in water
Pd(II)@Pyr:βꢀCD was also found to be a highly active and efficient catalyst in Heck reaction
of a wide variety of aryl bromides, containing substituents such as nitro, chloro etc., to couple
°
with styrene resulting in very high conversion and yield at 80 C. The reaction was optimized
and the observed results are given in Table 5. The reaction carried out without any ligand
afforded only 10% of desired product (entry 1). The reaction yield increased to 35% when
carried out with βꢀCD as the ligand in DMF (entry 2). Interestingly, when carried out with Nꢀ
octylpyridineꢀ2ꢀamine (2) in DMF, the reaction yield increased to 42% (entry 3). The reaction
when carried out with Pyr:βꢀCD (3) as a ligand in DMF became faster (entry 4). The reaction
failed in the absence of a base (entry 5). Among the different inorganic bases used for this
coupling reaction (entries 6, 7 and 8), K CO gave the best results. Using various solvents such
2
3
as DMSO, MeOH, CH CN and water, only moderate yields (entries 9ꢀ12) were obtained.
3
The efficiency of catalyst Pd(II)@Pyr:βꢀCD was also checked in coupling of various
substituted aryl halides with styrene. The results (Table 6) showed that different 4ꢀsubstituted
aryl bromides containing either electronꢀdonating or electronꢀwithdrawing groups such as
methyl, methoxy, nitro, cyano, aldehyde or acetyl group gave their analogous products in good
yields at short reaction times (Table 6, entries 1ꢀ8). We further investigated the Heck reaction of
heterocyclic bromides like 5ꢀbromopyridinꢀ2ꢀamine, 1ꢀ(5ꢀbromothiophenꢀ2ꢀyl)ethanone and 2ꢀ
bromopyridine with styrene (entries 9ꢀ11) and all the compounds gave the desired products in
good yields, though, the yields were lesser compared to aryl halides with electronꢀdonating and
electronꢀwithdrawing group substitutions. Heck coupling was also studied with two aryl
chlorides (Table 6, entries 12 and 13). The reactions, however, were slower and required longer
time.
1
0

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Table 5 Optimization of reaction conditions in Heck reaction
°
b
Entry Ligand
Catalyst
Base
Solvent Time (h)
Temp ( C) Yield (%)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
ꢀ
Pd(OAc)2 K CO
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
12
12
12
12
12
12
12
12
110
110
110
110
110
110
110
110
110
110
110
110
80
12
2
3
3
3
3
βꢀCD
Pyr
Pd(OAc)2 K CO
34
2
Pd(OAc)2 K CO
37
2
Pyrꢀ β CD Pd(OAc)2 K CO
51
2
Pyrꢀ β CD Pd(OAc)2
ꢀ
3
Pyrꢀ β CD Pd(OAc)2 Cs CO
72
2
3
Pyrꢀ β CD Pd(OAc)2 K PO
81
3
4
Pyrꢀ β CD Pd(OAc)2 Na CO
79
2
3
Pyrꢀ β CD Pd(OAc)2 K CO
DMSO 12
MeOH 12
MeCN 12
87
2
3
3
3
3
3
3
0
1
2
3
4
5
6
7
Pyrꢀ β CD Pd(OAc)2 K CO
71
2
Pyrꢀ β CD Pd(OAc)2 K CO
76
2
Pyrꢀ β CD Pd(OAc)2 K CO
Water
Water
Water
12
12
95
2
Pyrꢀ β CD Pd(OAc)2 K CO
95
2
Pyrꢀ β CD Pd(OAc)2 K CO
12, 6, 1, 0.5 80
95, 95, 95, 95
95, 95, 94, 89
95, 68, NR
10, 95, 95, 95,
2
c
d
e
Pyrꢀβ CD Pd(OAc)2 K CO3
Water 0.5
80
80
80
2
Pyrꢀ β CD Pd(OAc)2 K PO4
Water
Water
0.5
0.5
3
Pyrꢀβ CD Pd(OAc)2 K PO4
3
95, 95
Reaction conditions: 4ꢀBromobenzaldehyde (1 mmol, 1 equiv.), styrene (1.2 mmol, 1.2 equiv.), base (2 mmol, 2 equiv.),
°
b
c
solvent (2 mL), catalyst (0.2 mol %), Ligand (3) (0.2 mol%), 80 C, 30 min. GC yield. Yield of the reaction carried out
d
with 50, 40, 30 and 20 mg of Pd/pyꢀCD respectively. Yield of the reaction carried out with 100, 50 and 0 mol% of base
e
respectively. Yield of the reaction carried out with 0 (0.5), 1 (0.5), 1 (0.3), 1 (0.2), 0.5 (0.5) and 0.2 (0.2) mol% of ligand
(
catalyst) respectively.
1
1

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Table 6: Heck coupling reaction of different aryl halides with styrene
b
Entry
1
Aryl halide
Product
(Code)
Yield (%)
96
2
3
4
95
94
96
5
6
95
96
7
8
9
95
96
90
1
0
88
93
1
1
1
1
2
3
c
78
c
80
Reaction conditions: Aryl halides (1 mmol, 1 equiv.), styrene (1.2 mmol, 1.2 equiv.), base (2 mmol, 2
°
b
c
equiv.), solvent (2 mL), catalyst (0.2 mol %), Ligand (3) (0.2 mol %), 80 C, 30 min. GC yield, Reaction
time 2h.
1
2

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On the basis of the above results, and also in accordance with previous literature
3
1
reports, a plausible mechanism as shown in Scheme 2 is proposed.
Scheme 2 Plausible mechanism for the Pd(II)@Pyr:βꢀCD catalyzed CꢀC coupling
reactions in water
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The Pd(II)@Pyr:βꢀCD complex is formed in situ through the addition of Pd(OAc) to
2
Pyr:βꢀCD (3), via coordination between the –OH groups of βꢀCD, ꢀNH group of Nꢀoctylpyridineꢀ
2
ꢀamine (2) and Pd(OAc) . This is supported by the observation of a strong peak at 1579.02 for
2
ꢀ
ꢀ
(
[M+Pd(OAc) +Cl ] ) (Figure. 1), which corresponds to the complex between PyrꢀβꢀCD (3) and
2
Pd(OAc) . Oxidative addition of aryl halides/styrene leads to a Pdꢀchelated complex, generating
2
intermediate (II) which undergoes dissociation of halides through the formation of cationic
complex (III) and (V). Next, the insertion of phenyl from phenylboronic acid through the
transmetallation reaction, and also the synꢀinsertion of the alkene through the carbometallation
reaction, providing (IV) and (VI), which undergoes reductive elimination to give rise to Suzuki
0
Miyaura/Heck coupling products and simultaneously the catalytic active Pd complex is
regenerated in situ from a hydridopalladium complex after reductive elimination by base.
Figure 2 ESIꢀMS spectra of intermediates (II), (IV) and (VI), discussed in scheme 2.
Control experiments to identify the intermediates and hence to lend support to the
proposed mechanism, were also carried out. The reaction mixture, after the addition of
successive reagents to the catalyst, was analysed by ESIꢀMS. Observation of characteristic ESIꢀ
MS spectra (Figure 2) for intermediates (II), (IV for Suzuki) and (VI for Heck) provides strong
experimental evidence for the proposed mechanism. To the best of our knowledge, similar ESIꢀ
MS data for the proposed intermediates (II), (IV) and (VI) were not reported so far in Suzuki
Miyaura and Heck reactions.
Control experiments were also carried out in Suzuki Miyaura reaction with Pd(OAc)2
or Nꢀoctylpyridineꢀ2ꢀamine (2) as the catalysts and the results (Table 7) showed that Suzuki
Miyaura coupling reactions were slow with both Pd(OAc) , or Nꢀoctylpyridineꢀ2ꢀamine (2)
2
alone. When the reaction were carried out with Pd(II)@Pyr:βꢀCD good yields were obtained in
short reaction time. We believe that the differences in catalytic efficiency among unsupported
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4

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palladium and Pd(II)@Pyr:βꢀCD may be due to the presence of βꢀCD into the catalytic system,
which functioned as an excellent phaseꢀtransferꢀcatalyst to accelerate the present aqueous
organic transformation.
a
Table 7. Control experiments of Suzuki Miyaura coupling reactions in water.
Entry
Pd(OAc)2
Nꢀoctylꢀpyridineꢀ2ꢀamine
2)
Pd(II)@Pyr:βꢀCD
(
b
b
b
Time (h)
Yield (%) Time (h) Yield (%)
Time (h)
Yield (%)
1
2
3
4
0.5
1
12
28
30
40
0.5
1
10
10
25
35
0.5
1
95
95
91
90
3
3
3
6
6
6
a
The reaction were carried out in water (2 mL) in the presence of aryl bromides (1 mmol, 1 equiv.), phenylboronic
acid or styrene (1.2 mmol, 1.2 equiv.), base (2 mmol, 2 equiv.), Pd(OAc) (0.2 mol %), Pyr:βꢀCD (3) (0.2 mol%), 80
2
°
b
C. GC yield.
From the green chemistry point of view, recycling is one of the major concerns in metal
2
9
catalysis. In contrast to traditional water soluble MꢀL complexes, recycling of the
Pd(II)@Pyr:βꢀCD species was efficient. The recyclability of the presence of Pd@Pyr:βꢀCD (3)
catalytic system (Figure 3), in Suzuki Miyaura CꢀC coupling reaction of aryl halides and
phenylboronic acid showed that more than six catalytic cycles can be performed without any
significant loss of its catalytic activity.
Recycling of homogenous catalyst of Pd(II)@Pyr:βꢀCD.
Figure 3. I) Representative UVꢀDRS spectra for Nꢀoctylꢀpyridineꢀ2ꢀamine (2) (a), Pd(II)@Pyr:βꢀCD (b),
th
reusability for Pd(II)@Pyr:βꢀCD in 6 run (c): (II) Dependence of yield on the amount of catalyst for
Suzuki Miyaura reactions.
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The recycling of catalyst Pd(II)@Pyr:βꢀCD was carried out as follows: After the
completion of the reaction, the catalyst Pd(II)@Pyr:βꢀCD was recovered easily from the filtrate
by washing with ethyl acetate followed by evaporation of the aqueous phase in vacuo. A detailed
schematic representation is given in figure 4.
Figure 4. Recycling procedure of the Pd(II)@Pyr:βꢀCD catalytic system: (1) in situ formation of the
catalytic species. (2) Addition of phenylboronic acid/styrene and aryl bromide. (3) Stirring for 30 min. at
8
0 °C. (4) Cooling down to 23 °C. (5) Extraction with Et O. (6) Evaporation of the solvent. (7) Reuse of
2
the aqueous phase containing Pd(II)@Pyr:βꢀCD in subsequent catalytic runs.
Table 8 Comparison table for catalyst performance of different supported catalysts in the CꢀC
coupling reactions.
S.No Catalyst (mol%)
Solvent
Temp
Time
(h)
Yield
(%)
Ref.
°
C
SuzukiꢀMiyuara coupling
a
1
2
PdꢀHPꢀαꢀCD (0.5ꢀ0.01)
PdNPs/bisꢀTriazolylferrocenyl–
CD (0.5)
H O
EtOH:H O
60
25
24
24
100
98
45
56
2
2
b
3
4
5
6
bisꢀMIC CD:Pd (0.5)
βꢀCD:Pd NPs (0.05)
αꢀCytepꢀCD:Pd (3×10 )
βꢀCDꢀimidazolium
EtOH
Water
Xylene
Diaxone
90
85
120
50
16
2
1
99
95
95
99
56
32
43
38
c
ꢀ9
6
salt:Pd(2.22×10ꢀ2 )
d
ꢀ7
7
8
9
1
1
1
1
1
1
PdLn@βꢀCD (1×10 )
CDꢀcapped PdNPs (0.6)
Water
Water
Water
50
Reflux
90
100
130
80
80
50
80
4
2
24
12
4
0.5
14
24
0.5
98
98
90
100
71
50
81
85
95
36
44
41
46
47
48
11
49
e
PdꢀβCD–GNS (0.2)
f
0
1
2
3
4
5
PdNPs/Ps (0.5)
H O/DMF
2
g
ꢀ4
PdNPs/Chitosan (4.4×10 )
Xylene
DMF
h
Pd(II) complex (0.6)
i
PdNPs/TMC
H O
2
Pdꢀbasiczeolites (0.3)
Pd(II)@PyrꢀβꢀCD (0.2)
EtOH/H O
2
Water
Present work
Heck coupling
1
1
1
1
6
7
8
9
PdNPsꢀβꢀCD (1.2)
PdꢀαHpCD (0.5)
PSꢀPdONPs (1)
Water
Water
Water
Water
90
80
80
2
98
96
14
29
50
51
52
53
24
20
24
j
RAMEꢀβꢀCDmodified Pd/C (0.5)
100
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6

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2
2
2
2
0
1
2
3
4
PdꢀMPTAꢀ1 (10)
Water
Reflux
80
80
100
110
6
14
0.5
5
92
54
55
k
Pd(OAc) , PABCD (10)
H O:DMF
98
95
99
98
2
2
Pd(II)@PyrꢀβꢀCD (0.2)
RAMEꢀβꢀCD, Na PdCl (0.02)
Pdꢀ[SalenꢀPy][Br] (1)
Water
Water: hexane
Present work
57
58
2
4
2
DMF
4
2
a
b
PdꢀHpꢀαꢀCD hydroxyl propylꢀαꢀCD capped palladium. BisꢀMICꢀCD:Pdbisꢀmesoionic carbine cyclodextrin capped
C
d
palladium. αꢀCytepꢀCD:Pdcavitend supported tetraꢀphosphine in cyclodextrin with Pd. PdLn@βꢀCD complex with
e
functionalized βꢀcyclodextrin. PdβꢀCDꢀGNSPdNPs on βꢀcyclodextrin functionalized graphene nanosheets.
f
g
PdNPs/Ps dispersed on cross linked polystyrene polymer. PdNPs/ChitosanPdNPs stabilized on amino modified
h
i
chitosanꢀSchiff base ligand. Pd(II) complex PdNPs stabilized by dendrimers. PdNPs/TMCPdNPs supported in
thiourea modified chitosan. PdONPs polymer supported palladium nano particles. PABCD per amine modified βꢀ
cyclodextrin.
j
k
The reaction conditions, activity and effectiveness of the various other catalysts
employed earlier for the CꢀC coupling reactions of phenylboronic acids/styrene and aryl/alkyl
halides are given in Table 8. Comparison of the results indicates that our catalytic system (entry
1
6) exhibits better catalytic activity. The reported systems require organic solvents (entries 1ꢀ6,
1, 13 and 15), higher reaction temperature (entries 5 and 10ꢀ12) and higher reaction time
1
(entries 1ꢀ12, 14 and 15).
Experimental section
General Methods
All reactions were carried out under aerobic conditions. All chemicals were used as
commercially available without further purification unless otherwise noted. NMR spectra were
1
13
recorded at 300 MHz on a Bruker spectrometer. All H NMR and C NMR spectra were
measured in DMSOꢀd and CDCl with TMS as the internal standard. FTꢀIR spectral analyses
6
3
were performed using a JASCO FT/IRꢀ410 instrument by the KBr pellet technique in the range
−
1
of 4000–500 cm . Electrospray ionization mass spectrometry (ESIꢀMS) analyses were recorded
in LCQ Fleet, Thermo Fisher Instruments Limited, US. ESIꢀMS was performed in negative ion
mode. XRF analyses were recorded in Horiba Scientific, XGTꢀ5200, Xꢀray analytical
microscope.
General procedure for synthesis of Nꢀoctylꢀpyridineꢀ2ꢀamine (2)
A solution of NaH (29.3 mmol) in 1,4ꢀdioxane (8mL) was added dropwise to a solution
of 2ꢀaminopyridine (14.7 mmol) in 1,4ꢀdioxane (8mL). The reaction medium was then heated at
0
9
0 C for 1h. A grey color was observed afterwards, a solution of 1ꢀClꢀOctane (14.7 mmol) in
0
1
,4ꢀdioxane (10mL) is added and the reaction medium was heated at 90 C for 48h. Color of the
mixture changed from grey to pale yellow. 1,4ꢀDioxane is then removed in vaccum and 125 mL
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7

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water is added. The product was extracted with 3×25mL of ethyl acetate, yellow color liquid was
observed which was analyzed by NMR spectroscopy (300 MHz, CDCl , T=300 K, TMS=0 ppm)
3
and ESIꢀMS.
Synthesis of Pyridinium Modified βꢀCyclodextrin (3)
Nꢀoctylꢀpyridineꢀ2ꢀamine 2 (0.66 mmol) was added to a solution of dried monoꢀtosylꢀβꢀ
CD (0.22 mmol) in 1 mL DMF. The reaction medium was heated at 90 °C for 48 h. the reaction
mixture was then cooled to room temperature and the product was precipitated with acetone (4
mL). The solid was collected by filtration and washed with acetone (2 x 2 mL). Pure product is
obtained via acetone diffusion in DMF. The solid was then dissolved in deionised water (12 mL)
ꢀ
and anion exchange from tosylate to chloride was performed using an Amberlite IRAꢀ410 Cl
resin to yield 3 as a light yellow solid (0.12 mmol, 55 %), which was analyzed by NMR
spectroscopy (300 MHz, CDCl , T=300 K, TMS=0 ppm), FTꢀIR and ESIꢀMS. Melting point
3
°
2
10 C. [Elemental Analysis: for C H ClN O : calculated values C, 48.58; H, 6.75; N, 2.06,
5
5
19
2
34
Found: C, 49.32; H, 7.01; N, 2.12, (Figure S8)].
General procedure for Suzuki cross coupling reaction between aryl halides and
phenylboronic acid (4aꢀq)
A reaction tube equipped with a magnetic stirring bar was charged with pyridinium
modified βꢀcyclodextrin (3) (0.003 mmol, 3 mol%) and Pd(OAc) (0.003 mmol, 3 mol%)
2
dispersed in water (2 mL) for 15 min. The phenylboronic acid (1.3 mmol) was added to the in
situ formed Pd(OAc) complex with constant stirring. Aryl bromide/iodide (1 mmol) was added
2
0
and the mixture was allowed to stir at 80 C under aerobic conditions for 30 min. After
completion of the reaction, the product was extracted with ethyl acetate, filtered, dried with
sodium sulphate, and concentrated under reduced pressure. The resulting crude product was
purified by passing it through a column of silica gel 60–120 mesh using petroleum ether/ethyl
acetate (9:1 ratio) as the eluent, affording coupling product 4a–q, which was analyzed by NMR
spectroscopy (300 MHz, CDCl , T=300 K, TMS=0 ppm), FTꢀIR and ESIꢀMS. After extraction
3
of the product, the pyridinium modified βꢀcyclodextrin (3) was present in the water layer, which
was subsequently reused.
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General procedure for Heck cross coupling reaction between aryl halides and styrene (6aꢀ
k)
A reaction tube equipped with a magnetic stirring bar was charged with pyridinium
modified βꢀcyclodextrin (3) (0.003 mmol, 3 mol%) and Pd(OAc) (0.003 mmol, 3 mol%)
2
dispersed in water (2 mL) for 15 min. The aryl halides (1 mmol) was added to the in situ formed
pyr:βꢀCD/Pd(OAc) complex with constant stirring and continued for 30 min (to complete
2
complexation) and then olefins (1.3 mmol) was added and the mixture was allowed to stir at 80
0
C at 30 min. After completion of the reaction, the product was extracted with ethyl acetate,
filtered, dried with sodium sulphate, and concentrated under reduced pressure. The resulting
crude product was purified by passing it through a column of silica gel 60–120 mesh using
petroleum ether/ethyl acetate (9:1 ratio) as the eluent, affording coupling product 4a–q, which
was analyzed by NMR spectroscopy (300 MHz, CDCl , T=300 K, TMS=0 ppm), FTꢀIR and
3
ESIꢀMS. After extraction of the product, the pyridinium modified βꢀcyclodextrin (3) was present
in the water layer, which was subsequently reused.
Procedure for recording ESIꢀMS for intermediates (II), (IV) and (VI)
A reaction tube equipped with a magnetic stirring bar was charged with pyridinium
modified βꢀcyclodextrin (3) (0.003 mmol, 3 mol%) and Pd(OAc) (0.003 mmol, 3 mol%)
2
dispersed in water (2 mL) for 15 min. The aryl halides (1 mmol) was added and the mixture was
0
allowed to stir at 80 C at 30 min. The reaction mixture was separated and its ESIꢀMS was
recorded immediately for intermediate (II). The phenylboronic acid (1.3 mmol) and olefins (1.3
mmol) were added separately to the reaction mixture, allowed to stir for another 30 min. The
reaction mixture was separated and its ESIꢀMS was recorded immediately for intermediates (IV)
and (VI).
Conclusions
A novel cyclodextrin derivative, Pyr:βꢀCD (3) bearing Nꢀoctylꢀpyridineꢀ2ꢀamine
backbone is developed. This Pyr:βꢀCD(3) functions as an efficient supramolecular ligand for
Pd(II) ion and the prepared catalyst, was characterized by NMR, Mass spectrometry, FTꢀIR
spectroscopy, UVꢀVisible spectroscopy and DLS (Dynamic Light Scattering). Using this
Pd(II)@Pyr:βꢀCD as a homogenous catalyst, an useful and improved aqueous protocol is
developed for SuzukiꢀMiyaura and Heck cross coupling reactions of a wide range of aryl halides
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(X= I, Cl, Br) and phenylboronic acid/styrene in water, and it can be used for six runs without
significant loss in its activity. The use of environmentally benign water medium, good and
excellent yields, easy and rapid recycling protocol, makes this reaction a valid candidate towards
the desired objectives of green chemistry. Consequently this catalyst system would open up
novel challenging designs of new water soluble catalyst systems with potential industrial
applications as well as synthetic utility.
Acknowledgements
KP thanks to CSIR, India for financial support, RI thanks to UGC and MKU for NonꢀNet
fellowship.
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